What is the rejection region in hypothesis testing?

• A. The area of the probability distribution cut off by alpha.
• B. The central region around the mean.
• C. The entire distribution.
• D. The set of all possible sample means.

Answer: (A)

In hypothesis testing, the rejection region is the part of the sampling distribution of the test statistic where results are unlikely enough under the null hypothesis to lead you to reject it. It’s defined by the significance level alpha, so under the null, only alpha of outcomes fall into this region. Practically, that region is the tail(s) beyond the critical values: in a two-tailed test you have two tails, in a one-tailed test you have one tail. The central region, by contrast, is where you would fail to reject the null. The entire distribution isn’t the rejection region, and the rejection region isn’t about all possible sample means—it's about which observed test statistic values trigger rejection.